Steel is the generic name for a group of ferrous metals, composed principally of iron, which have considerable durability and versatility. By the proper choice of carbon content, addition of alloying elements, and by suitable heat treatment, different kinds of steel can be made for various purposes and the use in industry of all kinds of steel is now quite expansive.
Stainless steel is defined as a steel alloy, with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as traditional steel. There are over 150 different grades and surface finishes to allow the stainless steel to suit the environment in which it will be used. Stainless steel's low maintenance and relatively low cost make it an ideal base material for many commercial applications. It is used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, it is also used as a structural alloy for cars and as a construction material for buildings.
Stainless steels have a passive film of chromium oxide that forms in the presence of oxygen due to the chromium present in the steel. This layer blocks most corrosion from spreading into the metal's internal structure. High oxidation resistance can be achieved with chromium additions of 13% by weight up to 26% for harsh environments. The chromium forms a passive layer of chromium III oxide (Cr2O3) when exposed to oxygen. To have their optimum corrosion resistance, stainless steel surfaces must be clean and have an adequate supply of oxygen to maintain this passive surface layer.
Cleaning of stainless steel includes the removal of various surface contaminants to ensure corrosion resistance, to prevent contamination, and to achieve the desired appearance of the steel. Acid cleaning is a process by which a solution of a mineral or organic acid in water sometimes in combination with a wetting agent or detergent or both, is employed to remove iron and other metallic contamination, light oxide films, soil and similar contaminants.
Acid cleaning compositions for removing contaminants from stainless steel generally have the mineral or organic acid in a solution with a pH of less than 7.0. The compositions typically remove both organic (dirt, oils) and inorganic (oxides, free iron) soils in the same operation. They also are used to improve corrosion resistance and enhance brightness of the base metal surface.
One of the problems which arises in the use of steel is its corrosion, either by the atmosphere or by the environment in which it is used. The rate of corrosion may vary, depending on the surrounding conditions and also the composition of the steel. Stainless steel, especially, is much more resistant to corrosion than plain carbon and other steels. This resistance is due to the addition of chromium to alloys of iron and carbon. Other metals, for example copper, and aluminum, also increase corrosion resistance but they are limited in their usefulness. Although stainless steel has appreciable resistance to corrosion, it will still corrode in certain circumstances and attempts have been made to prevent or reduce this corrosion. Most acid cleaners also include a corrosion inhibitor of some sort. For example, in acid media copper sulphate has been used as a corrosion inhibitor. However this and other proposed inhibitors are not entirely satisfactory since, like copper sulphate, they may be expensive, introduce an effluent disposal problem and, moreover, are not entirely effective. For example, when copper containing urea sulfate solutions are placed in contact with nickel metal, copper will plate the nickel surface.
A variety of compounds, including dialkylthioureas, such as diethylthiourea and dibutylthiourea, are known to reduce the corrosivity of sulfuric acid to carbon steels. Thioureas are not appropriate for food and beverage situations as any remnant thiol compounds are considered contamination for such surfaces.
The type of acid used has also presented problems in development of acid cleaners. Most acid cleaners are based upon phosphoric acid due to its low cost, good combining ability with other ingredients, its ability to exert a very strong synergistic cleaning affect, and its low corrosivity.
Phosphoric acid has recently fallen out of favor due to environmental concerns. It is therefore one object of this invention to provide a phosphate free acid cleaning composition which has equal or superior cleaning and corrosive inhibiting properties as phosphoric acid based cleaners.
It is another object of this invention to provide aqueous, urea sulfate based acid cleaning compositions which are relatively noncorrosive due to addition of gluconic acid, to stainless steel and which reduce the cost of such cleaners.
Yet another object is to provide a liquid acid cleaning composition for stainless steel which is biodegradable and which includes components which are generally recognized as safe.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims.